In this podcast, Gabriel Oria shows us how to create ringtones using Amadeus Pro for Mac.
Amadeus Pro lets you use your Mac for any audio-related task, such as live audio recording, creating iPhone ringtones, digitizing tapes and records, converting between a variety of sound formats (including Mp3, MPEG-4 AAC, AIFF, WAVE, WMA, CAF, FLAC, Ogg Vorbis), remastering your iTunes music library, record internet streams, etc.
There is also a Lite version of Amadeus available at a lower price and with less features. However, the process for creating ringtones would be the same as that demonstrated in this podcast.
Trial versions of both editions of Amadeus are available from the developer's website.